1. Field of the Invention
The present invention relates to a method of forming a thin film for a semiconductor device, and more particularly, to a method of forming a thin film using an N.sub.2 O gas for improving of characteristic of the thin film. Although the present invention has a wide range of applications, it is particularly suitable for use as a gate insulating film or a dielectric film of a capacitor.
2. Discussion of the Related Art
A thermal oxide film is generally used as a gate insulating film in a semiconductor device. However, as the pattern dimension of the semiconductor device becomes smaller and miniaturized, the thermal oxide file has several problems. For example, leakage current increases due to an electric field and interface state density increases, (see K. Naruke, S. Taguchi, and M. Wada IDEM 1988, pp 424).
To improve such problems, a method is provided for improving hot carrier characteristic and electric integrity in a gate oxide film using an oxide film including nitrogen. However, the characteristic of the gate oxide film varies depending on how the nitrogen is incorporated into the oxide film. In earlier used oxide film, a thermal oxide film was nitride treated or nitrated to an ammonia gas and the electron capture characteristics increased by adding H.sub.2 into the oxide film. In addition, degradation of electron mobility occurred by an excessive nitrogen content (see T. Ito, T Nakamura, and H. Ishikawa, IEEE Trans. Electron Devices, 1982, ED-29, pp498).
An N.sub.2 O oxide film, which has been studied since the late 1980's, has an excellent hot-carrier effect and an excellent electrical characteristic. However, since the nitrogen incorporated into the interface between the oxide film and the silicon has a small content of 1-2 Atomic %, it is not appropriate to use the nitrated oxide film as the interface layer of boron diffusion (see G. W. Yoon, A. B. Joshi, J. Kim, and D. L. Kwong; IEEE EDL 1993, Vol. 14, pp179).
A conventional method of forming the thin film (used as an insulating film or a dielectric film) of the semiconductor device will be described with reference to the accompanying drawings.
First, the method of forming the thin film used as the insulating film of the semiconductor device will be described referring to FIG. 1a and FIG. 1b which are sectional views showing process steps of a conventional gate insulating film.
The conventional insulating film, as shown in FIG. 1a and FIG. 1b, includes a silicon substrate 1, an oxide film 2, and a thermal oxide film 3. The oxide film 2 on the silicon substrate 1 is removed through a furnace of high temperature in the ambient of O.sub.2 or H.sub.2 O before forming a gate electrode. Then, thermal oxide film 3 is grown. To incorporate the nitrogen into the interface between the silicon substrate and the thermal oxide film in the thermal oxidation process, either the N.sub.2 O gas is used as an oxidation reaction material or the thermal oxide film is nitrated to N.sub.2 O gas or NH.sub.3 gas.
The formation of the thin film used as the dielectric film will be described below with respect to a DRAM.
A DRAM generally includes a stack capacitor and a trench capacitor. The stack capacitor includes a fin structure, a cylinder structure, and a box structure. The cylinder structure is generally used taking the capacitance into consideration.
For high capacitance, it is suggested that a contact area of a storage node electrode and a plate electrode made large in a limited cell area and that a dielectric film be formed of a material having a high dielectric ratio. A Ta.sub.2 O.sub.5 thin film, which is often used in the capacitor of the DRAM due to its high dielectric ratio, has a large leakage current at As deposition. For such a reason, it is not suitable for a dielectric material of the capacitor in a highly integrated device such as 256M DRAM. Therefore, an appropriate annealing is required to reduce the leakage current.
At present, it is regarded that N.sub.2 O annealing is more effective than O.sub.2 annealing using a high temperature furnace and rapid thermal oxidation(RTO) for reducing the leakage current. It is reported that optimal condition of rapid thermal annealing(RTA) using the N.sub.2 O gas can be achieved by annealing for 60 seconds at a temperature of 800.degree. C. (see S. C. Sun and T. F. Chen IDEM 94-333, 1994).
The formation of the dielectric film in the DRAM capacitor having the widely used cylinder structure will now be described below with reference to FIGS. 2a-2d which are sectional views of process steps showing a dielectric film in a capacitor according to the prior art.
As shown in FIG. 2a, a field oxide film 11 (for device isolation), impurity diffused regions 12, gate electrodes 13, bit lines 14, and insulating films 15a, 15b, 15c, and 15d are formed on a semiconductor substrate 10. A storage node electrode 16 contacts one side of the impurity diffused regions 12 on the semiconductor substrate 10. The insulating films 15a, 15b, 15c, and 15d insulate and flatten respective layers and are used as end point in an etching process. The storage node electrode 16 includes a polysilicon.
Subsequently, as shown in FIG. 2b, the surface of the storage node electrode 16 is treated by rapid thermal nitridation(RTN) using NH.sub.3 gas.
As shown in FIG. 2c, Ta.sub.2 O.sub.5 having a high dielectric ratio is deposited on the entire surface of the RTN treated storage node electrode 16 by chemical vapor deposition(CVD) so as to form a dielectric film 17. The dielectric film 17 is then treated by RTN at a temperature of 800.degree. C. with a pressure of 760 Torr for 60 seconds using the N.sub.2 O gas.
As shown in FIG. 2d, a plate electrode 18 is formed by depositing TIN or the like.
As aforementioned, the electrical characteristic of the thin film used as an insulating film or a dielectric film can be improved by the method for forming the thin film using N.sub.2 O gas. However, although the conventional method for forming the thin film enables electrical characteristic to be improved to a certain extent, it has several problems in its application as the pattern dimension site decreases. First, the problems caused by using the thin film as an insulating film will be described.
In case of using a pure thermal oxide film or a oxide film including nitrogen as the gate insulating film, the pattern dimension is reduced. As a result, the gate insulating film becomes thinner. If the thickness of the gate insulating film becomes 50 .ANG. or lower, direct tunneling occurs instead of F-N tunneling in the tunneling characteristic of the electron. The direct tunneling causes the characteristic of the gate insulating film to deteriorate and thus, becomes unsuitable for a device.
In addition, the conventional gate insulating film has a disadvantage, that its insulating characteristic deteriorates by implanting ion such as boron during forming of p.sup.+ poly gate over the gate insulating film in a later process.
The problems caused by using the thin film as a dielectric film will be described.
Ta.sub.2 O.sub.5 dielectric film treated by RTA for 60 seconds at the temperature of 800.degree. C. in the atmospheric pressure has more excellent leakage current characteristic than the dielectric film treated by RTO annealing or O.sub.2 annealing in a high temperature furnace. However, it has a problem in that the effective thickness of the dielectric film becomes larger. For this reason, the film is difficult to use despite its improved leakage current characteristic.